Short answer: You were right.

Linux is used on more web servers that Microsoft, it's network layer is pretty mature and stable.
I didn't assume an error in the Linux's networking code, but some kind of silly misconfiguration on my computer.

We are talking about Knoppix run right from the disc and not Knoppix run from the hard disk, are we not? I pitty the fool who installs Knoppix to hard disk, but I don't get involved in trouble shooting the many problems it introduces.
Oh, no, I am indeed running Knoppix from the hard disk. But I almost haven't changed anything, I have only installed the zd1211 WLAN driver which worked flawlessly.
I am using Knoppix for the disk installation because of its good hardware detection; with other distributions I had to spend a lot of time to get all the hardware configured. Why do you say that installing Knoppix on the hard disk creates so many problems?

Back to my problem: Everything seemed to work well, but it didn't. For example the WLAN driver is loaded without problems, DHCP seemed to work (no error, and a suitable IP address was assigned), and it was also possible to ping my own IP address (not 127.x.y.z, but the address associated to the wlan0 device). Using ethereal I discovered that although all that seemed to work, even DHCP just reported an old IP address. (As far as I remember, on other Linux installations there was always a delay of several seconds and then an error message if DHCP didn't work, this was not the case here.) No error messages at all, although not a single byte could be transmitted.

Thus, the problem is not in the network layer, but somewhere below. I guess the USB system does not work correctly. The USB mouse makes no problems, but the WLAN stick seems to be more delicate. Even worse, the behaviour of the USB system is not really deterministic; sometimes it workes better, sometimes worse, so I have to reboot. I have had these problems already with a previous Knoppix installation, but with the new Knoppix 4.0 it seemed to work much better, but it is the opposite. I was indeed able to establish a connection to the internet over my WLAN stick, but it lasted only a minute or so (yes, ethereal did show a handful IP packets). Of course there was no error message, but suddenly the wlan0 device had disappeared, and after bringing it up again the network did not work anymore, even after rebooting the computer several times. Maybe the saved settings (e.g. DHCP) come from such a situation when the WLAN accidentially worked for a short time.

I have found some websites saying that there are problems with some VIA USB chipset and the kernel 2.6 (and that the chipset is to blame). As there is no firmware update available for my BIOS, I will buy a new USB controller and see if things get better afterwards. So thank you for your help; it looked like a network problem but it seems to be somewhere deeper...

Christoph